Chapter 8: Insulation


If your roof is being replaced during your loft conversion, it’s a good time to add some insulation, and even if the whole roof isn’t being replaced, insulation is still required. There is a minimum standard that’s acceptable by the Building Regulations, but if you want to reduce your heating bills, choose good standard or best standard insulation instead.


If the existing roof covering is coming off as part of your loft extension, you can put insulation over the rafters with a breathable roofing to ventilate the roof area, giving what is called a ‘warm roof’. It is usually allowed to have an increase in the height of the roof to 150mm for insulation.

The manufacturer’s instructions must be followed with insulation but, depending on the type of roof and the type of insulation, gaps may need to be left for air space. Check the Building Regulations for details. If rafters are only 100mm deep, battens should be added to their underside to maintain a 50mm air gap above the insulation.

If the roof is not being replaced, insulation will be required between and beneath the rafters This will reduce the height of the ceiling, so it’s best to use the thinnest insulation materials possible.

Types of Insulation

There are many different types of insulation, but here are the seven main ones:

1. Rigid board (PUR/PIR)

PUR insulation is a rigid closed-cell low‑density insulation and PIR is less combustible and has higher working temperature limits.

2. Extruded polystyrene

Extruded polystyrene is produced from structured, closed-cell foam so it keeps its final structure and thermal values.

3. Mineral wool (glass/stone)

Mineral wool insulation is made from a range of materials including recycled glass and silicate fibres.

4. Multi-foil

Multi-foil insulation has layers, separated by wadding and foam which are sewn together to form a thin insulating blanket.

5. Expanded polystyrene

Expanded polystyrene is cut or shaped and mouldings manufactured, used in suspended beam and concrete floors.

6. In situ applied PUR/PIR

PUR and PIR insulation can also be applied straight to any surface to be insulated.

7. Phenolic foams

Phenolic foam can be up to 50% more efficient than other insulation materials and this allows a smaller thickness to be used, which saves space.

That’s quite a baffling array of options so, if you have any questions, call our qualified experts on 0800 046 1995.